Environmental Engineering Reference
In-Depth Information
3.3.4
L IQUID -L IQUID E QUILIBRIUM
3.3.4.1
Octanol-Water Partition Constant
If two liquid phases are in contact, and a solute is present in both, then at equi-
librium we have a distribution of solute between the two phases consistent with
equal chemical potentials or fugacity values. In environmental engineering, liquid-
liquid equilibrium is common: for example, the distribution of organic chemicals in
the water environment where a third phase (e.g., oil) is present, such as occurring
during oil spills at sea and inland waterways, floating oils in wastewater treatment
plants, and sub-surface spills in contact with groundwater. Solvent extraction is a
well-known operation in environmental engineering separation processes. Since the
concept of like dissolves like is mostly true, it should be expected that most organic
compounds would have a greater affinity for organic solvents and substrates. A spe-
cific liquid-liquid system (octanol-water) has special relevance to environmental
engineering.
Consider a solute i that is distributed between two solvents (octanol
o and water
w). At equilibrium the solute i should have equal fugacity in both phases. Thus
f i
f i ,
=
(3.47)
the total number of moles of i in the system. We have
x i γ
o
i f lo
x i γ
w
i f lo
=
,
(3.48)
i
i
and therefore
x i
w
i
x i = γ
K ow =
(3.49)
o
i
γ
is the partition constant for a solute between two phases defined as the ratio of mole
fractions.
A large body of literature exists on the partitioning of a variety of environmen-
tally significant compounds between the organic solvent, 1-octanol, and water (Leo,
Hansch, and Elkins, 1971). The octanol-water partition constant defined in terms of
the ratio of the molar concentrations of solute i in both phases is designated K ow :
γ
V w
V o
,
x i V w
w
i
γ
C i o
C i w =
K ow =
x i V o =
(3.50)
o
i
where V w and V o are the partial molar volumes of water and 1-octanol, respectively.
The availability of such a large database on K ow is not entirely fortuitous. It has
long been a practice in pharmaceutical sciences to seek correlations of the various
properties of a drug with its K ow . 1-Octanol appears to mimic the lipid content of biota
verywell.Asimilarreasoningledtoitsacceptanceasadescriptorofchemicalbehavior
in the environment. 1-Octanol has the same ratio of carbon to oxygen as the lipids
and represents satisfactorily the organic matter content in soils and sediments. It is
 
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